Low NOX burners

Over 90% of the NOx from a typical flame is in the form of NO and the remainder is NO2.

NOX is controlled in a typical burner using

1. Staged fuel combustion
2. Fuel gas recirculation
3. Lean premix combustion
4. Air flow pattern

Good article here.

Furnace bridge wall

Bridge wall in a furnace is the section the radiation section ends and the convection section begins. The function of the bridge wall is to force the flue gas in a particular path and concurrently generate a pressure drop which affects the draft through the furnace as a whole. The temperature at that point is the bridge wall temperature.

Single fired and double fired furnaces.

In single fired, burner in the middle and tubes on the either sides.In double fired furnaces the tubes are in the middle and burners on both sides, thereby heated (fired) from both sides.

Double fired

Single fired